1. Field of the Invention
The present invention relates to a method and apparatus for machining a deep hole, and more particularly to a method and apparatus for machining a deep hole which, when machining a deep hole in a workpiece with a gun drill, can optimize the supply of a coolant according to the machining stage.
2. Background Art
A drill is generally used in machining of a hole in a workpiece, and a gun drill is widely employed to machine a deep hole in a workpiece. For example, when machining a mold for molding of a large-sized resin product, such as an instrument panel or a bumper of an automobile, deep hole machining with a gun drill is carried out to machine a cooling hole or a slide core hole for insertion of an extrusion pin.
Deep hole machining with a gun drill have specific problems: For example, a coolant becomes harder to reach the tip of the gun drill and discharge of shavings becomes difficult as a hole becomes deeper.
In machining of a deep hole with a gun drill, it is a conventional method to allow a coolant to pass through a passage extending axially within the gun drill and jet at a high pressure from the tip of the gun drill, thereby supplying the coolant to a machining point and discharging shavings.
According to conventional practice, the high-pressure coolant is blown on a workpiece from the start of machining of a deep hole, and the pressure of the coolant is kept constant during the machining. This involves the problem of too much scattering of the coolant upon its collision with the workpiece as well as the problem of decrease in the effect of discharging shavings with the progress of machining to a deeper hole. Especially in the case of a deep hole to be machined in a large-sized mold such as for use in molding of an instrument panel or a bumper of an automobile, it is necessary to use a long gun drill, for example having a length of not less than 70 mm, making it difficult to discharge shavings.